Composition and process for brightening aluminum and its alloys



Jan. 23, 1962 Filed Jan. 26, 1959 E. M. DUKE, JR COMPOSITION AND PROCESS FOR BRIGHTENING ALUMINUM AND ITS ALLOYS 2 sheets-sheet 1 O//VU9/V//7lg/7Qj-t' Q/V//V/BS m k l Q53 Q Nk w kl g n MN u lllgxg @www u Q Qw n Q j Q QSQ @w k @u l w L/ I@ QINNN @DUNES /l s Q Q Q m l l\ Q Q k w wLL fg 'j H s INVENTOR.

Dweo M. oaef, JQ

Jan. 23, 1962 E, M. DUKE, JR 3,018,211

COMPOSITION AND PROCESS FOR BRIGHTENING ALUMINUM AND ITS ALLOYS FiledJan. 2e, 1959 2 Sheets-Sheet 2 cally mentioned. One or more than one ofthese comF plex fluoride ions can be present. The source materials forthese complex iiuorides can be, for example, the alkali metal orammonium salts of the complex uorides or any sufliciently solublecomplex fluoride salts of a metal which does not form insoluble materialwith any of the other anions present in the solution, such as thehexavalent chromium-containing anion.

Specic illustrative examples of such complex uorides are sodium,potassium or ammonium uoborates and silicouorides, potassium titaniumfluoride, sodium titanium uoride, potassium zirconium fluoride, ammoniumberyllium fluoride, ammonium cadmium iiuoride, ammonium aluminum uoride,ammonium chromium uoride, potassium molybdenum uoride, potassium rheniumfluoride, ammonium zirconium uoride, potassium zirconium pentauor-ide,sodium zirconium pentaiiuoride, potassium fenic uoride, potassium zincfluoride, ammonium titanium fluoride, potassium hafnium uoride,potassium columbium fluoride, and potassium tantalum uoride.

in the case of the AlF4 complex anion, this can be formed either fromthe double salt, eg., alkali metal or ammonium aluminum fluoride, orfrom the simple aluminum uoride which in the nitric acid solution of theinvention forms the complex AlF4 ion. However, where AlF3 is used as thesource of both aluminum ions and complex iiuoride ions, because of thetendency of some aluminum fluoride to form the aforementioned A1114ions, thus reducing the concentration of free of disassociated aluminumions, it is preferred to employ alu-minum uoride in conjunction withother aluminum salts, e.g., aluminum sulfate, -to supply sufncientconcentration of aluminum ions. The concentration of complex fluorideions in the solution is preferably about 0.01 to about 0.10 mol perliter of solution. It has been found particularly advantageous to employa Weight ratio of free aluminum ions to complex uoride ions of about 2to 1.

The pH of the brightening solutions of the invention having theaforementioned ingredients, is usually about 0.5 or less, and in mostcases about 0.4 or less. The temperature of the solutions is preferablymaintained between 180 and about 210 F., although lower temperatures canbe employed. The employment of such elevated temperatures not onlyserves to increase the eiiiciency of operation and reduce time oftreatment, but lalso aids in conjunction with the complex fluoride ion,and the aforementioned concentration of nitric acid, to maintain insolution the relatively high concentration of aluminum ion required forproper functioning of the bath. Time of treatment in my brightening bathis usually from about 5 to 20 minutes, depending on temperature of thebath, the concentration of the specific components thereof, and the ageof the bath. 1I-t will be understood that the temperature and time oftreatment are not critical and can vary from the aforementioned ranges.

The compounds serving as the source for the essential ions of the brightdip composition of the invention, can be added separately lto a watersolution in amounts sulficient to form the acid bath having thepreferred aforementioned ranges of said ions, or alternatively, a solidcomposition composed of a mixture of two or more of said compounds in-proper proportions can be added to an aqueous nitric acid solution inproper concentration to form the bright dip bath.

When processing aluminum parts to be brightened according to theinvention, it is first preferred to remove sols from the surface of theparts by treatment in a precleaning composition, such as, for example, asolution formed by 'dissolving in Water a composition composed of 95%tetrasodium pyrophosphate, 3% sodium chromate and 2% sodium dodecylbenzene sulfonate, in a concentration of 6 ounces of the com-positionper gallon of solution. The parts should then be rinsed thoroughly incold or warm running water to remove any residue from 4 the parts. lfdesired, the parts can then be pre-etched in a caustic alkali solution,eg., 10% NaOH solution, for a few minutes. When such pre-etching isemployed, this tends to increase the tank `life of the bright dip bathof the invention.

The aluminum parts can then be ideoxidized by treatment, e.g., for 2 to5 minutes, in a deoxidizer of the type disclosed in U.S. applicationSerial No. 710,403 of Isidore Pollack, tiled January 22, 1958. Anillustrative bath of this nature is composed of 91% sodium bisulfate, 8%chromic acid (CrO3) and 1% potassium titanium fluoride dissolved inwater in an amount of 8 ounces of said composition per gallon ofsolution. The parts are then rinsed thoroughly in cold running water.

The parts are then treated in the bright dip bath of the invention,preferably under the pH conditions, temperature and time period notedabove. Gentle mechanical agitation o fthe Work pieces in the bath, oragitation of the bath itself, enhances the specular brightness. However,excessive agitation is to be avoided, especially in aged baths, as ittends to accelerate graining therein. To prolong the bath lige, it isrecommended that about 1.0% of the bath be discarded for each 10 sq. ft.of aluminum processed per working gallon of solution.

After the aluminum parts have been brightened according to theinvention, the parts are rinsed thoroughly in cold, agitated, overowingwater. The Work surface can then be passivated in a chromic acidpassivating solution, eg., 0.5% chromic acid (Cr03) solution, adjustedto pH of 2.0 with nitric acid, for a short period, e.g., 10 to 20seconds, to form a thin inert layer of oxide on the brightened aluminumsurface without adversely affecting the specular brightness thereof.

It will be understood that treatment in any one or more of theaforementioned auxiliary treating baths, that is,

pre-cleaner, pre-etching, deoxidizer or passivating baths,

is optional, and can be omitted if desired.

It has been found that treatment in the bright dip bath of the inventionproduces a highly polished mirror-like surface on commercial aluminum,e.g., 1100 and 3003 aluminum, and on alloys such as 2024 and 6061, abright smooth surface of high specular reflectance is obtained, which isnot quite as reflective as the surface formed on 1100 or 3003 aluminum.However, the bright dip bath hereof is effective on various aluminumalloys in addition to those specifically mentioned above.

The following examples illustrate practice of the invention and theadvantages thereof:

EXAMPLE l Two solutions A and B are made up as follows:

Percent by Weight Aluminum nitrate (Al(NO3):.0H2O). Water 89. 8

Solution A contains 1.27 mol hydrogen ion, 1.27 mol nitrate ion, .15 molhexavalent chromium and .039 mol TiPS ion, based on a liter of solution.Solution B contains 1.27 mol hydrogen ion, 1.39 mol nitrate ion, .l5 mollhexavalent chromium, .039 mol TiFG ion and .14 mol aluminum ion, basedon a liter of solution.

A number of panels of 3003 aluminum were treated in solution A at 200 F.and a number of other panels of the same alloy were treated in solutionB maintained at the same temperature as solution A.

The square feet of aluminum processed in each of `solutions A and B andthe corresponding remaining strength or acidity of the bath as percentby weight nitric acid are determined at various intervals. These valuesare plotted, giving the curves shown in the FIG. l of the accompanyingdrawing. The horizontal line indicated at X and corresponding to aconcentration of about 5.2% by weight nitric acid represents an averageconcentration noted herein as the transitional concentration, abovewhich polishing and/ or brightening occurs and below which the polishingand brightening action appears to decrease substantially and grainingoccurs. The greater the distance above the line X, the higher thespecular reflectance produced on the part surface.

From the plots shown in the drawing, it is seen that using solution Anot containing aluminum nitrate initially, the bath strength, i.e., itsacidity in terms of nitric acid concentration, decreases rapidly and thebath reaches the transitional concentration when less than 8 square feetof aluminum per gallon of solution is processed. When using solution Bof the invention, containing the required concentration of aluminumions, together with the required concentration of nitrate and hexavalentchromium, about l5 square feet of aluminum surface per gallon ofsolution are processed before the transitional concentration is reached,showing the eiectiveness of the presence of such aluminum ions inincreasing the life and elliciency of the bright dip. Moreover, solutionB can be revivied by addition of nitric acid to process say 30 squarefeet or more of aluminum per gallon of solution before the solution isexhausted, whereas with addition of the same amount of nitric acid tothe solution A the bath is exhausted when only 16 to 20 square -feet ofaluminum per gallon of solution are processed.

.-Further, at points above the transitional concentration indicated byline X on each of curves A and B, for any given value of square feet ofaluminum processed per gallon of solution, the specular reflectance ofthe aluminum treated with solution B containing the requisiteconcentration of aluminum ions, is superior to that for solution A. Forexample, at the value on the abscissa corresponding to processing of sixsquare feet of aluminum in each of the baths, the specular reflectanceof parts processed in solution B is materially greater than that forsolution A. As the number of square feet of aluminum processed pergallon of solution increases, the greater is the difference between thehigh specular reilectivity produced in solution B as compared to theinferior brightening action of solution A. Also, processing of aluminumparts in solution B in which more than 8 and less than 15 square feet ofaluminum per gallon of solution have previously been processed continuesto give a high polished surface, whereas after processing about 8 squarefeet of aluminum per gallon of solution using solution A, furtheroperation proceeds in the region below the transitional concentration,giving inferior results, including graining.

During the processing of the aluminum panels in each of solutions A andB, as the nitric acid is consumed, aluminum from the panels is dissolvedin the solution, increasing the aluminum ion concentration. However, inthe case of solution A the nitric acid is consumed much faster than insolution Bof the invention, placing substantially more aluminum intosolution than solution B. When between 6 and 8 square feet of aluminumhave been processed in solution A, there is a large amount of aluminumion present substantially in excess of the 0.20 mol per liter upperlimit for the concentration of aluminum cation according to theinvention, and such excess aluminum ion functions as a contaminant inthe bath. About 13 to 14 square feet of aluminum can be processed insolution B before it becomes contaminated with an excess of aluminum ioncomparable to the excess aluminum ion present when only 6 to 8 squarefeet of aluminum have been processed in solution A. Hence it is apparentthat bath A becomes quickly contaminated by excess aluminum ion due torapid consumption of nitric acid, whereas bath B of the inventioncontaining a predetermined concentration of aluminum ion initially insolution reduces the consumption of nitric acid and can be used forprocessing many more square feet of aluminum before it becomescontaminated to an equal degree with excess aluminum ion, as compared tobath A.

FIGURE 2 of the drawing is a plot of square feet of aluminum processedper gallon of solution against percentage of contaminants in the form ofexcess aluminum, precipitated and reduced chromates and othercontaminants including iluoride precipitates, formed during thereaction.

Curve A represents the contamination level of bath A and curve B thecontamination level of bath B. Thus, for example, when 8 square feet ofaluminum has been processed using solution A, the contamination level isabout 4.2%, and according to FIGURE 1, at this point bath A is no longereffective as a brightening bath. On the other hand, at 8 square feet ofaluminum processed in solution B, the contamination level for bath Bcontaining aluminum ion initially is only about 2.7%, and at this levelof contamination, bath B is still effective as a brightener. Bath B doesnot reach the 4.2% contamination level until about 13 square feet ofaluminum has been processed.

EXAMPLE 2 The following solutions C to K are prepared, a practical rangeof proportions of the components which can be used also being given foreach of the specific compositions:

Table I Percent by Weight C Range 3. 60 to 12.0 s Iso to 4I 0o 8 26 to 130 Remainder emainder 8 3 3 8 26 Remainder 16 to .8O .26 to 1. 3 8O to 63.6 to 12.6 Remainder 1100, 3003, 2024 and 6061 aluminum alloys aretreated in each of the compositions C to K noted above for a period ofabout minutes, the temperature ofreach of the solutions being maintainedat about 200 F.

In each case, the respective parts treated in the above solutionsdevelop a smooth, polished bright surface of high specular reilectance,with absence of pitting and graining, and with substantially no loss ofbase metal. However, best results are noted with compositions containingtitanium, zirconium and aluminum complexes with iluorine, that is,compositions C to G and K. Particularly with the 1100 and 3003 aluminumsamples, the compositions of Table I produce mirror-like surfaces. Ofthe compositions E and F, wherein the uorine complex is the AlF4 ion,composition F containing additional free aluminum ion furnished by thealuminum sulfate produces better results. In composition G, the NaA1O2in nitric acid solution breaks down to give aluminum nitrate.

From the foregoing, it is seen that the invention provides improvednovel compositions and procedure for eiiicient brightening and polishingof aluminum, Without accentuating the formation of smut or grain, andwith substantially no base metal loss.

The term consisting essentially of as used in the deiinition of theingredients present in the composition claimed is intended `to excludethe presence of other materials in such amounts as to interferesubstantially with the properties and characteristics possessed by thecomposition set forth but to permit the presence of other materials insuch amounts as not substantially to atiect said properties andcharacteristics adversely.

The term aluminum employed in the claims is intended to denotecommercially pure aluminum and aluminum alloys.

While I have described particular embodiments of my invention forpurposes of illustration, it should be understood that variousmodications and adaptations thereof may be made within the spirit of theinvention as set forth in the appended claims.

I claim:

1. A chemical brightener bath for aluminum, which comprises an acidaqueous solution consisting essentially of about .60 to about 2.00 molsper liter of hydrogen ions, about :60 to about 3.00 mols per liter ofnitrate ions, a concentration of hexavalent chromium-containing ionequivalent to about .05 to about 0.40 mol per liter of hexavalentchromium, about .01 to about .10 mol per liter of complex fluoride ionsselected from the group consisting of the SiFG, TiF6, ZrF, BF., and AlF4ions, and about .02 to about .20 mol per liter of aluminum cations, thepH of said solution being not greater than about 0.5.

2. A chemical brightener bath as defined in claim 1, wherein saidcomplex fluoride ions are TiFG ions.

3. A chemical brightener bath for aluminum, which comprises an acidaqueous solution consisting essentially of nitric acid, aluminumnitrate, chromic acid and potassium titanium fluoride, the amounts ofsaid ingredients present being such as to provide about .60 to about2.00 mols per liter of hydrogen ions, about .60 to about 3.00 mols perliter of nitrate ions, chromate ions in a concentration equiv-alent toabout .05 to about 0.40 mol per liter of hexavalent chromium, about .01to about .10 mol per liter of uotitanate ions, and about .02 to about.20 mol per liter of aluminum cations, the pH of said solution being notgreater than about 0.5.

4. A chemical brightener bath for aluminum, which comprises an acidaqueous solution consisting essentially of nitric acid, aluminum uorideand chromic acid, the amounts of said ingredients present being such asto provide about .60 to about 2.00 mols per liter of hydrogen ions,about .60 to about 3.00 rnols per liter of nitrate ions, chromate ionsina concentration equivalent to about .05 to about 0.40 mol per liter ofheXav-alent chromium, about .01 to about .10 mol per liter of AIF.,ions, and about .02 to about .20 mol per liter of aluminum cations, thepH of said solution being not greater than about 0.5.

5. A process for chemically brightening aluminum, which comprisestreating said aluminum in an aqueous acid solution consistingessentially of about .60 to about 2.00 mols per liter of hydrogen ions,about .60 to about 3.00 mols per liter of nitrate ions, la concentrationof hexavalent chromium-containing ion equivalent to about .05 to about0.40 mol per liter of hex-avalent chromium, about .01 to about .10 molper liter of complex fluoride ions selected from the group consisting ofthe SiF, TiFe, ZrFG, BF., and AlF4 ions, and about .02 to about .20 molper liter of aluminum cations, the pH of said solution being not greaterthan about 0.5.

` 6. A process as dened in claim 5, wherein the temperature of saidsolution being maintained between about and about 210 F.

7. A composition which consists essentially by weight of .50 to 6.0parts of a soluble hexavalent chromium-containing compound, 0.1 to 2.0parts of a soluble complex uoride of the group consisting of solublesilicoliuondes, titanium liuorides, zirconium uorides, uoborates andaluminum uorides; and .16 to 3.8 parts of a soluble aluminum salt, saidcomposition when added to an aqueous nitric acid solution having anitric acid concentration furnishing about .60 to about 3.00 mols perliter of nitrate ions, forming =a brightening bath for aluminum,comprisf ing hexavalent chromium-containing ion equivalent to about .05to about 0.40 mol per liter of hexavalent chromium, about .01 to about.10 mol per liter of complex fluoride ions selected from the groupconsisting of the Sil-76, TiFs, ZrF, BF., and AIF., ions, and about .02to about .20 mol per liter of aluminum cations, and about .60 to about2.00 mols per liter of hydrogen ions, said bath having a pH not greaterthan about 0.5.

References Cited in the rile of this patent UNITED STATES PATENTS

1. A CHEMICAL BRIGHTENER BATH FOR ALUMINUM, WHICH COMPRISES AN ACIDAQUEOUS SOLUTION CONSISTING ESSENTIALLY OF ABOUT .60 TO ABOUT 2.00 MOLSPER LITER OF HYDROGEN IONS, ABOUT .60 TO ABOUT 3.00 MOLS PER LITER OFNITRATE IONS, A CONCENTRATION OF HEXAVALENT CHROMIUM-CONTAINING IONEQUIVALENT TO ABOUT .05 TO ABOUT 0.40 MOL PER LITER OF HEXAVALENTCHROMIUM, ABOUT .01 TO ABOUT .10 MOL PER LITER OF COMPLEX FLUORIDE IONSSELECTED FROM THE GROUP CONSISTING OF THE SIF6, TIF6, ZRF6, BF4 AND ALF4IONS, AND ABOUT .02 TO ABOUT .20 MOL PER LITER OF ALUMINUM CATIONS, THEPH OF SAID SOLUTION BEING NOT GREATER THAN ABOUT 0.5.
 5. A PROCESS FORCHEMICALLY BRIGHTENING ALUMINUM. WHICH COMPRISES TREATING SAID ALUMINUMIN AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF ABOUT .60 TO ABOUT2.00 MOLS PER LITER OF HYDROGEN IONS, ABOUT .60 TO ABOUT 3.00 MOLS PERLITER OF NITRATE IONS, A CONCENTRATION OF HEXAVALENT CHROMIUM-CONTAININGION EQUIVALENT TO ABOUT .05 TO ABOUT 0.40 MOL PER LITER OF HEXAVALENTCHROMIUM, ABOUT .01 TO ABOUT .10 MOL PER LITER OF COMPLEX FLUORIDE IONSSELECTED FROM THE GROUP CONSISTING OF THE SIF6, TIF6, ZRF6, BF4 AND ALF4IONS, AND ABOUT .02 TO ABOUT .20 MOL PER LITER OF ALUMINUM CATIONS, THEPH OF SAID SOLUTION BEING NOT GREATER THAN ABOUT 0.5.